Milk or juice is typically packaged in cartons that have been sterilized to prolong the shelf life of the contents under refrigeration. When milk or juice is packaged under aseptic packaging conditions, the contents are capable of being stored for a substantial period of time at room temperature without spoilage. Both of these packaging processes require effective sterilization of the interior of the carton before being filled.
Aseptic packages containing milk or juice may be stored at room temperature for substantial periods of time because the bacteria which normally produces spoilage has been killed in the packaging process. Various methods and apparatus have been developed for packaging milk and juice under aseptic conditions. For example, U.S. Pat. No. 4,375,145 discloses an aseptic packaging machine having a conveyor on which preformed cartons advance under ultraviolet germicidal lamps to expose the interior of the cartons to ultraviolet (UV) radiation. In addition, the interior of the cartons may be sprayed with a germicidal solution, such as hydrogen peroxide, before passing under the ultraviolet lamps.
The use of high intensity lamps necessitates incorporating a fast shuttering system for safety reasons and to prevent overheating of the cartons. During normal operation, the UV lamp is enclosed in the filling machine which prevents exposure of the operator to Uv light rays. If the filling machine jams or if for some reason the operator must open the doors to the filler, then there must be some mechanism to minimize exposure to the UV light. The UV light can be either turned off or shuttered. Turning off the light requires a lengthy start-up time whereas shuttering provides protection for the operator with no loss of time upon restarting.
U.S. Pat. No. 4,289,728 discloses a method for sterilization of the surfaces of food containers and other materials by applying a hydrogen peroxide solution, followed by ultraviolet radiation. This patent indicates that the peak intensity of ultraviolet radiation occurs at a wavelength of 254 nm. The concentration of the hydrogen peroxide solution is less than 10% by weight, and furthermore, the hydrogen peroxide solution is heated during or subsequent to irradiation.
Current technology utilizing ultraviolet (UV) sterilization of cartons is limited by the low intensity of the UV lamps that can be used. UV output in the range of 0.1 to 1 W/cm.sup.2 has previously been considered to be a "high intensity" source for sterilization of packaging (Maunder, 1977). Low power lamps in the 0.1 to 1.0 W/cm.sup.2 can be convection cooled and are effective in sterilizing flat surfaces in close proximity to the lamp.
Recent developments in the area of high output medium pressure mercury UV lamps have increased the light output to 50-250 Watts per inch of bulb length (17-85 Watts/cm.sup.2). This type of lamp has a long cylindrical quartz glass tube containing medium pressure mercury vapor with electrodes at the opposite ends of the tube. The high power consumption of these lamps necessitates utilization of an active cooling system to prevent overheating of the lamp and to be able to restart the lamp after it has been temporarily shut down. Cooling systems generally consist of a thimble of quartz surrounding the lamp through which air or water is circulated.
UV sterilization has been shown to be suitable for sterilization of flat films but has limited applicability to preformed, angular containers (Maunder, 1977) due to the geometric and physical constraints associated with UV light. If a simple UV lamp is placed in close proximity above a preformed container, such as a gable top carton, the sterilization effectiveness is severely limited due to several reasons. The total light flux entering the carton is restricted to light that can be directed through the carton opening, which in the case of typical gable top cartons are 55.times.55 mm, 70.times.70 mm or 95.times.95 mm. Light emitted from a line source UV lamp decreases in intensity with the square of the distance from the light source. Thus, as the depth of the carton increases, the light intensity falls off dramatically.
Another problem in sterilizing these cartons with UV light is that the light enters the top of the carton and radiates toward the bottom substantially parallel to the sides of the carton. The germicidal effect of the light that impinges on the sides is very low because of the low angle of incidence. Thus, the sides of the cartons are the most difficult surfaces to sterilize, especially for tall cartons. When the cartons are positioned on the conveyor, two sides of the carton lie in a plane that is parallel to the axis of the lamp, while the other two sides are transverse to the axis of the lamp. Since the lamp is elongated, radiation impinges on the transverse sides of the carton at a higher angle of incidence than it does on the parallel sides of the carton. In the case of a single UV lamp source above the center of a 70.times.70.times.250 mm rectangular carton, the effective light intensity at the bottom of the carton would be reduced to 13.9% of the maximum intensity at that distance from the source. The carton sides transverse to the lamp axis receive light from the entire length of the bulb. Light originating from the lamp reflector on the side opposite the parallel carton wall will have a maximum incident angle and thus have an intensity equal to 27.0% of the lamp intensity.
A typical arrangement for a cylindrical UV light system has a single-mirrored lamp in a water-cooled sleeve placed in a shuttered, reflective housing. This arrangement is suitable for sterilization of flat surfaces and some shallow cartons but the intensity of the light falls rapidly with increasing distance from the bulb, so that it is not suitable for sterilizing tall cartons.
Although these prior methods and apparatus produce satisfactory results for flat films, they are neither effective nor efficient when used for sterilizing preformed cartons.